Hide/unhide rows based on form input

Maybe try something like:

Private Sub OK_Click()
 Sheets(1).Activate
Rows("17:17").Hidden = Not contactmelder.Value
Rows("18:18").Hidden = Not gasdetector.Value
Rows("19:19").Hidden = Not rookmelder.Value
Rows("20:20").Hidden = vochtdetector.Value
Rows("21:21").Hidden = bewegingsmelder.Value
Rows("22:22").Hidden = trekkoord.Value
Rows("23:23").Hidden = handzender.Value
Rows("24:24").Hidden = valdetector.Value
Rows("25:25").Hidden = alarmeringshorloge.Value
Range("A15,A16,A26").EntireRow.Hidden = WorksheetFunction.Subtotal(102, Rows("17:25")) = 0
End Sub

althought it assumes that the rows in question contain some data and are not just blank.